The present invention relates to a diagnosis apparatus for an evaporation system, and particularly, to an apparatus for precisely diagnosing a leak of evaporation gas in the evaporation system in which an evaporation gas (evaporation fuel) generated in a fuel tank is adsorbed in an adsorbent within a canister, and the adsorbed fuel is purged into an intake system of an internal combustion engine under the predetermined operating conditions for combustion.
The conventional evaporation system for an engine has the airtight construction in order to prevent the evaporation gas from being purged into the atmosphere. However, in the case where the passage for the evaporation gas in the evaporation system is broken or the pipe is disconnected for some reasons, the evaporation gas in the canister becomes purged into the atmosphere. Further, also in the case where the purge passage connected to the intake pipe of the engine or the like becomes clogged, this obstructs the purge of the evaporation gas.
It is necessary for coping with the inconvenience of the evaporation system to diagnose in advance the presence or absence of generation of troubles of the evaporation system. In the technique disclosed in Japanese Patent Laid-Open No. Hei 6-193518, intake negative pressure is taken into the evaporation system through the purge valve, the pressure change of the evaporation system is detected by a pressure sensor, and the trouble of the evaporation system is detected and diagnosed on the basis of the pressure change.
Further, Japanese Patent Laid-Open No. Hei 6-249095 discloses the diagnosis by a pressure sensor for detecting pressure of a fuel tank. Inspection of a liquid quantity of the fuel tank is executed, and a duty ratio of vent valve control of the tank is decided on the basis of fuel obtained. Thereafter, the vent valve is opened and the cutoff valve is closed at the decided duty ratio, and the diagnosis of a leak of the evaporation gas is carried out from a negative pressure reducing gradient of negative pressure which reduces in the tank.
In the technique for introducing negative pressure into the evaporation system to diagnose the evaporation system according to the pressure change thereof, as in the conventional diagnosis of the evaporation system as described above, the pressure change in the evaporation system is affected by the residual quantity of fuel in the fuel tank (relating to the space volume), the atmospheric density (which changes with the altitude or the like) or the like, resulting in an error of detection. The present applicant has already proposed diagnosis apparatuses in which a gauge valve is arranged in an evaporation system for precisely performing diagnosis without being affected as described above (see Japanese Patent Laid-Open No. Hei 8-35452/U.S. Pat. No. 5,575,265 and Japanese Patent Laid-Open No. Hei 9-203352). In these proposals, the pressure change is changed to a pressure change in an evaporation system in the state where a gauge valve is opened and closed and the gauge valve having a predetermined vent area is opened or a pressure change in an evaporation system in the state where the gauge valve is closed to thereby cancel the aforementioned influence to precisely perform the diagnosis of the evaporation system.
However, the diagnosis apparatus using the gauge valve as described above has a problem in that if stain or the like should adhere to the gauge valve to reduce the vent area, an error would occur in the result of diagnosis.
Further, it is assumed that an opening of a gauge pipe is at the atmospheric pressure. However, for example, in the case where the gauge pipe is communicated with an intake pipe of the engine, when stain or the like adheres to an air cleaner to generate the pressure loss, an error also occurs in the result of diagnosis.
The present invention has been accomplished in view of the problem as described above. It is an object of the present invention to provide a diagnosis apparatus for an evaporation system in which even if stain adheres to a gauge valve of the evaporation system or an air cleaner for intake air, the diagnosis of the evaporation system can be carried out precisely.